1. Field of the Invention
The present invention relates to a photosensitive composition suitable as a positive-type image forming material and also to a planographic printing plate precursor using the composition. More specifically, the present invention relates to a positive-type image forming composition that is designed for use with an infrared laser and is characterized by writability with heat, particularly with an infrared laser, thermal head or the like, and by suitability for use in a planographic printing plate precursor that can be prepared by a so-called direct plate production process in which the plate is prepared directly according to digital signals, particularly from a computer or the like. Further, the present invention relates to a planographic printing plate precursor using the foregoing composition.
2. Description of the Related Art
In recent years, due to progress in technologies related to solid-state lasers and semiconductor lasers emitting rays in regions ranging from a near-infrared region to an infrared region, a system that uses these infrared lasers in a printing plate production process so that a printing plate is prepared directly according to digital data from a computer, has been drawing attention.
Japanese Patent Application Laid-Open (JP-A) No. 7-285,275 discloses a material for a positive-type planographic printing plate precursor for use with an infrared laser in direct plate production. This invention relates to an image forming material comprising a resin soluble in an aqueous alkaline solution, a substance capable of generating heat by absorbing light, and a positive-type photosensitive compound such as a quinone diazide, wherein in image portions, the positive-type photosensitive compound acts as a dissolution inhibiting agent which substantially reduces the solubility of the resin soluble in the aqueous alkaline solution, whereas in non-image portions, the positive-type compound is thermally decomposed to thereby lose its dissolution inhibiting capability and can be removed through developing.
As a result of their investigations, the present inventors have found that a positive image can be obtained without using a quinone diazide in an image recording material. However, mere elimination of the quinone diazide from the image recording material presents a drawback in which the stability of sensibility with respect to the concentration of a developer, i.e., latitude in development, becomes poor.
On the other hand, an onium salt or a compound capable of forming alkali-insoluble hydrogen bonds is known to act as an agent that inhibits an alkali -soluble polymer from becoming dissolved in an alkali. As to an image forming material for use with an infrared laser, WO 97/39,894 describes that a composition that uses a cationic, infrared-ray absorbing dye as an agent that inhibits an alkali-soluble polymer from being dissolved in an alkali exhibits a positive-type operation. This positive-type operation in an operation in which the infrared-ray absorbing dye absorbs the laser light and generates heat which causes the polymeric film to lose a dissolution-inhibiting effect in irradiated region, to thereby form an image.
However, this invention is associated with a problem that, although the image-forming performance of the photosensitive composition at a surface thereof irradiated with a laser is satisfactory, a satisfactory effect cannot be obtained in the depths of the composition because a sufficient amount of heat is not diffused to the depths, and, as a result, an on-off distinction between exposed areas and unexposed areas is insufficient in a developing process with alkali. Consequently, a good image is not obtained (i.e., low sensitivity and narrow latitude in development). The term xe2x80x9clatitude in developmentxe2x80x9d as used herein means the broadness of the allowable range of the alkali concentrations of an alkali developer for the creation of a good image.
Further, infrared-ray absorbing agents are generally used in order to improve the sensitivity of image forming materials. As an infrared-ray absorbing agent used for image forming materials, for example, the following are known: dyes such as cyanine dyes or metal complexes, and pigments such as carbon black or phthalocyanine. However, metal complexes are problematic since their molecular extinction coefficients are low, and pigments are problematic since they tend to cause blemishes during development. Cyanine dyes impair development when they are used in excess though they are suited for the purpose in terms of wavelengths and molecular extinction coefficients.
The inventors of the present invention found that the use of an anionic oxonol dye provides good images and as a result filed Japanese Patent Application No. 10-79912. However, more improvement regarding wavelengths that are apt for infrared-rays was desired.
As an example of an oxonol dye that absorbs wavelength in a long wavelength region, EP No. 444789 discloses pentamethine compounds. However, EP No. 444789 includes no description of physical properties such as the molecular extinction coefficient or the like although they are important factors for an image forming material. EP No. 397435 discloses examples of heptamethineoxonol dyes. However these compounds have a problem in that their absorption wavelengths are 780 nm or less and thus insufficient for corresponding to an IR laser. Additionally, their molecular extinction coefficients are small.
As is discussed above, there are very few known oxonol dyes whose maximum absorption wavelength are near or over 800 nm, and therefore an anionic, infrared-ray absorbing agent whose absorption wavelength is in a long wavelength region has been desired.
Accordingly, an object of the present invention is to provide a novel anionic, infrared-ray absorbing agent having an absorption wavelength is in a long wavelength region and capable of being preferably used for an image forming material, to provide a photosensitive composition characterized by high sensitivity, good latitude in development, and good storage stability, and to provide a positive-type planographic printing plate precursor that uses the foregoing composition in a direct plate production process and enables an image to be formed with high sensitivity with an infrared laser.
After a series of studies for the purpose of upgrading image-forming performance, i.e., increasing sensitivity and latitude in development, the present inventors found that latitude in development was improved by use of specific, anionic, infrared-ray absorbing agents, and filed a patent application, i.e., Japanese Patent Application No. 10-79,912, based on this finding. As a result of further studies, they discovered that a better match with a laser as well as superior sensitivity and broad latitude in development can be obtained by using, inter alia, an oxonol-based dye having a relatively long conjugated chain in a molecule thereof as the anionic, infrared-ray absorbing agent, and they have achieved the present invention based on this finding.
Further, among these anionic infrared-ray absorbing agents, the compound represented by the following general formula (1) is a novel compound with naphthoindandione groups in its end groups, which provides a large molecular extinction coefficient and an absorption wavelength in a long wavelength region.
Namely, a photosensitive composition of the present invention comprises the following components (a) and (b) and becomes soluble in an aqueous alkaline solution when irradiated with an infrared laser:
(a) an anionic, infrared-ray absorbing agent; and
(b) a polymeric compound insoluble in water but soluble in an aqueous alkaline solution.
Preferably, the (a) the anionic, infrared-ray absorbing agent of the photosensitive composition of the present invention is (a-1) an anionic metal complex, (a-2) an anionic carbon black, (a-3) an anionic phthalocyanine, or (a-4) a compound represented by the following general formula (I):
{Gaxe2x88x92xe2x80x94Mxe2x80x94Gb}m Xm+xe2x80x83xe2x80x83(I) 
wherein, M represents a conjugated chain of carbon atoms; Gaxe2x88x92 represents an anionic substituent group; Gb represents a neutral substituent group; and Xm+ represents a cation including a proton, said cation having a valence of from 1 to m, wherein m represents an integer ranging from 1 to 6.
Further, it is preferable that (a) the anionic, infrared-ray absorbing agent of the photosensitive composition of the present invention is (a-4) the compound represented by the following general formula (1): 
wherein, L represents a conjugated methine chain which has seven or more carbon atoms and which may contain substituent groups that may form a ring by linking together; X+ represents a cation; Y1 to Y6 and Z1 to Z6 each independently represents a hydrogen atom or a substituent group such that substituent groups thereof may form a ring by linking together.
Further, it is preferable that the counter cation of (a) the anionic, infrared-ray absorbing agent has a construction of a thermally decomposable onium salt.
The planographic printing plate precursor of the present invention comprises a substrate and a photosensitive layer formed thereon comprising the foregoing photosensitive composition.
Although the working mechanism of the photosensitive composition of the present invention is not clear, the mechanism is presumably as follows. The use of (a) the anionic, infrared-ray absorbing agent having a relatively long conjugated chain causes the counter cation to act as a dissolution inhibitor of (b) the polymeric compound soluble in alkaline water and the dissolution-inhibiting effect can be eliminated by heat, which is generated when the anionic, infrared-ray absorbing agent absorbs the laser light, in portions irradiated with the laser. Because the anionic, infrared-ray absorbing agent had a relatively long conjugated chain that comprises 7 or more carbon atoms in a molecule thereof, an improvement in sensitivity was achieved, presumably because the anionic, infrared-ray absorbing agent had a high molecular absorption coefficient in a near infrared region and excellent solubility in a solvent.
In a case of conventionally known cationic, infrared-ray absorbing dyes, for the purpose of absorbing light in an infrared region, the molecular structure of the dye is inevitably confined within a limited scope so that the structure that has the dissolution-inhibiting effect, will not inhibit the light absorption in the infrared region. Therefore, the dissolution-inhibiting effect cannot be controlled as desired by the molecular structure. In contrast, when the anionic, infrared-ray absorbing agent is used as in the present invention, since the counter cation can be selected freely and since the dissolution inhibition effect can be controlled at will, image-forming performance can be enhanced as desired, which is an effect of the present invention. In addition, the use of the anionic, infrared-ray absorbing agent having a relatively long conjugated chain, such as heptamethine or nanomethine, in the anionic portion increases the molecular absorption coefficient in the near infrared region, thus improving further the adaptability to a laser. Among these anionic, infrared-ray absorbing agents, the novel compound of the present invention represented by theral formula (1) has naphthoindandione groups in its end groups and therefore has a large molecular extinction coefficient and an absorption wavelength shifted towards longer wavelengths.
Furthermore, the use of a thermally decomposable onium salt as the counter cation of (a) the anionic, infrared-ray absorbing agent enables an image to be formed with an even higher level of sensitivity and broader latitude in development, presumably because the thermally decomposable onium salt is decomposed by the heat generated by the infrared absorbing agent to thereby eliminate the dissolution-inhibiting effect more rapidly and with a higher level of sensitivity.